1. Field of the Invention
The present invention generally relates to special purpose data processing circuits and, more particularly, to data processing circuits for image data in automated inspection systems.
2. Description of the Prior Art
Image processing for display generation or feature measurement, extraction or recognition is perhaps the most computationally and storage intensive category of data processing problem commonly encountered at the present time. Feature measurement, such as in measurement of cell features in biological applications and the monitoring of coating or layering processes are two examples of applications of automatic feature measurement which require high-speed computation of high accuracy feature measurement data. While the actual processing may be quite simple in some cases, such problems typically involve relatively massive amounts of data. For example, consider that the smallest unit of an image for processing is referred to as a pixel with which the image surface is tiled, often in the form of a matrix. If the resolution of the image is a matrix of one thousand pixels on a side, the complete image will include one million pixels. Each pixel may further contain data representing images values such as color, hue and saturation and other data which represents the relationship to other pixels such as an object number or a location in a three-dimensional scene. The number of bits of such image value information must be multiplied by the number of pixels in the image to obtain the number of bits necessary to represent the image. Since some operation may be required on any or all of this image data, it can be understood that many millions of data processing operations are required for even modest degrees of spatial and image value resolution.
Accordingly, many approaches have been tried in recent years to reduce the amount of data which must be processed in order to achieve desired levels of throughput from image processing apparatus. Nevertheless, even with high-speed computers, possibly including special purpose co-processors and pipelined architectures, it is not uncommon for the processing of a single image to require several hours to complete. In the field of automated inspection systems, such processing times present a major limitation on throughput of manufacturing processes. However, in some manufacturing fields, such as the manufacturing of electronic circuit devices at high integration densities, there is no viable alternative to automated inspection.
Therefore, the art of image feature measurement, extraction and/or recognition has generally advanced through the development of special purpose image transducers and processing arrangements which are specifically adapted to particular kinds of image features. For example, U.S. Pat. No. 4,424,588 to Satoh, describes processing to detect the position of a symmetrical article. U.S. Pat. No. 4,499,597, to Alves, describes centroid accumulation for small object detection. Pixel values are compared among adjacent pixels to determine the maximum image value pixel in a character segment in U.S. Pat. No. 4,625,330, to Higgins and a similar neighborhood comparison is disclosed in Gennery, U.S. Pat. No. 4,703,513; both being directed to the enhancement of video signals. An arrangement for imaging a three-dimensional device applied to lead frame assembly is disclosed in U.S. Pat. No. 5,030,008, to Scott et al. An optical system for distance measuring is disclosed in U.S. Pat. No. 5,054,926, to Dabbs et al. Some exemplary data processing arrangements for use in image processing systems are also disclosed in U.S. Pat. Nos. 5,016,173, to Kenet et al, 4,918,636, to Iwate et al., 4,963,018, to West, 4,979,221, to Perryman et al., 4,925,302, to Cutler, 4,845,356, to Baker, 4,818,110, to Davidson, and 4,707,610, to Ludlow et al.
This latter patent to Ludlow et al. is directed to the measuring of surface profiles and line width measurements in regard to the manufacture of integrated circuit devices. The wafer to be measured is mounted for oscillatory movement and an optical system focusses a beam on a small spot on the surface. The spot is scanned along the wafer while the focus is progressively changed to derive a series of samples of the surface profile.
More recently, however, it has become desirable to measure a plurality of reflecting profiles within a body of material such as would be presented by a plurality of layers of a semiconductor structure. In such imaging, it may not be possible to, say, follow each profile separately and differentiation of profiles may be difficult. Also, since multiple scanning is relatively slow and potentially could engender positional errors in connection with the different surfaces imaged, it is desirable to sense all profiles in a single scan of the object to be imaged.
At the present state of the art in optical transducers (e.g. electronic cameras), spatial resolution over the surface (e.g. the x and y directions) of the device and within (e.g. in the z direction) the device is on the order of one-quarter micron. Therefore, even a small surface such as that of a chip may involve several million pixels and the profile information may desirably reach the resolving of, say, eight surfaces at eight bits dimensional accuracy. Bit streams of data from each surface are essentially derived simultaneously, in parallel, for each pixel. Accordingly, it is seen that an extremely great quantity of data must be captured, stored and processed to exploit the capabilities of transducers presently available and to accomplish the desired imaging.
Consider also that some attempts to increase processing speed have involved truncation of data. However, such truncation effectively discards information which is present in the original signals from a transducer device. Even truncation is of extremely limited value in reducing data since it is now common to produce integrated circuits having several millions of components, each of which must be imaged with sufficient resolution for meaningful inspection, requiring several pixels for each component or feature of a component. Therefore, data truncation, even to one bit per pixel, is very much limited by the minimum amount of data which is required to achieve the desired inspection function.
In summary, the present state of the data processing art does not allow full exploitation of the capabilities of optical transducers now possible or allow real time processing of such large amounts of data as are required for a desired degree of optical resolution and with sufficient throughput to provide an automated, real time, inspection system suitable for present manufacturing systems for high density integration electronic components.